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4 Rules in Brainstem Localisation
I hope this will be as useful as it was for me in localising lesions within the brainstem. In short, you can divide the brainstem into latitudes and longitudes. The latitudes are determined by the cranial nerve nuclei, and the longitudes are determined by "pathways" (eg. spinothalamic tract, corticospinal tract) Blood supply Firstly, a quick review of the blood supply of the brainstem. Simply put the blood supply comes from: #paramedian branches #long circumferential branches (SAP) *superior cerebellar artery (SCA) *anterior inferior cerebellar artery (AICA) *posterior inferior cerebellar artery (PICA) And occlusion of these two groups of vessels results in two distinct types of brainstem syndrome: #medial (or paramedian) brainstem syndromes ( due to para-median branch occlusion) #lateral brainstem syndromes ( due to occlusion of the circumferential branches, also occasionally seen in unilateral vertebral occlusion) Graphical representation The 4 Rules of Brainstem #There are 4 structures in the ‘'midline'‘ beginning with M''' #There are '''4 structures to the ‘'side'‘ (lateral) beginning with S''' #There are '''4 cranial nerves in the medulla, 4''' in the '''pons '''and '''4 above the pons (2 in the midbrain) #The 4 motor nuclei that are in the midline are those that divide equally into 12 except for 1 and 2, that is 3, 4, 6 and 12 (5, 7, 9 and 11 are in the lateral brainstem The 4 medial structures and the associated deficits are #'Motor pathway' (or corticospinal tract): contralateral weakness of the arm and leg #'Medial Lemniscus': contralateral loss of vibration and proprioception in the arm and leg #'Medial longitudinal fasciculus': ipsilateral inter-nuclear ophthalmoplegia (failure of adduction of the ipsilateral eye towards the nose and nystagmus in the opposite eye as it looks laterally) #'Motor nucleus and nerve': ipsilateral loss of the cranial nerve that is affected (3, 4, 6 or 12) The 4 ’side’ (lateral) structures and the associated deficits are: #'Spinocerebellar pathway': ipsilateral ataxia of the arm and leg #'Spinothalamic pathway': contralateral alteration of pain and temperature affecting the arm, leg and rarely the trunk #'Sensory nucleus of the 5th cranial nerve': ipsilateral alteration of pain and temperature on the face in the distribution of the 5th cranial nerve (this nucleus is a long vertical structure that extends in the lateral aspect of the pons down into the medulla) #'Sympathetic pathway': ipsilateral Horner’s syndrome, that is partial ptosis and a small pupil (miosis) The 4 Cranial Nerves and the associated deficits are: The 4 cranial nerves in the medulla are CN9-12: #Glossopharyngeal (CN9): ipsilateral loss of pharyngeal sensation #Vagus (CN10): ipsilateral palatal weakness #Spinal accessory (CN11): ipsilateral weakness of the trapezius and stemocleidomastoid muscles #Hypoglossal (CN12): ipsilateral weakness of the tongue The 12th cranial nerve is the motor nerve in the midline of the medulla. Although the 9th, 10th and 11th cranial nerves have motor components, they do not divide evenly into 12 (using our rule) and are thus not the medial motor nerves. The 4 cranial nerves in the pons are CN5-8: #Trigeminal (CN5): ipsilateral alteration of pain, temperature and light touch on the face back as far as the anterior two-thirds of the scalp and sparing the angle of the jaw. #Abducent (CN6): ipsilateral weakness of abduction (lateral movement) of the eye (lateral rectus). #Facial (CN7): ipsilateral facial weakness. #Auditory (CN8): ipsilateral deafness. The 6th cranial nerve is the motor nerve in the medial pons. The 7th is a motor nerve but it also carries pathways of taste, and using the rule of 4 it does not divide equally in to 12 and thus it is not a motor nerve that is in the midline. The vestibular portion of the 8th nerve is not included in order to keep the concept simple and to avoid confusion. Nausea and vomiting and vertigo are often more common with involvement of the vestibular connections in the lateral medulla. The 4 cranial nerves above the pons are CN1-4: #Olfactory (CN1): not in midbrain. #Optic (CN2): not in midbrain. #Oculomotor (CN3): impaired adduction, supradduction and infradduction of the ipsilateral eye with or without a dilated pupil. The eye is turned out and slightly down. #Trochlear (CN4): eye unable to look down when the eye is looking in towards the nose (superior oblique). The 3rd and 4th cranial nerves are the motor nerves in the midbrain. Thus a medial brainstem syndrome will consist of the 4 M’s and the relevant motor cranial nerves, and a lateral brainstem syndrome will consist of the 4 S’s and either the 9-11th cranial nerve if the lesion is in the medulla, or the 5th, 7th and 8th cranial nerve if the lesion is in the pons. Handy tip: If there are signs of both a lateral and a medial (paramedian) brainstem syndrome, then one needs to consider a basilar artery problem, possibly an occlusion.